Letter pubs.acs.org/macroletters
Colloidal Random Terpolymers: Controlling Reactivity Ratios of Colloidal Comonomers via Metal Tipping Nicholas G. Pavlopoulos,† Jeffrey T. Dubose,† Erin D. Hartnett,† Kookheon Char,‡ and Jeffrey Pyun*,†,‡ †
Department of Chemistry and Biochemistry, University of Arizona, Tucson, Arizona 85721, United States Department of Chemical and Biological Engineering, Program for Chemical Convergence for Energy and Environment, and the Center for Intelligent Hybrids, Seoul National University, Seoul 151-744, Korea
‡
S Supporting Information *
ABSTRACT: We report on a versatile synthetic method of preparing colloidal copolymers and terpolymers composed of dipolar Au@Co core−shell nanoparticles (NPs) in the backbone, along with semiconductor CdSe@CdS nanorod (NR), or tetrapod (TP) side chain groups. A seven-step colloidal total synthesis enabled the synthesis of well-defined colloidal comonomers composed of a dipolar Au@CoNP attached to a single CdSe@CdS NR, or TP, where magnetic dipolar associations between Au@CoNP units promoted the formation of colloidal co- or terpolymers. The key step in this synthesis was the ability to photodeposit a single AuNP tip onto CdSe@CdS NR or TP that enables selective seeding of a dipolar CoNP onto the AuNP seed. We show that the variation of the AuNP size directly controlled the size and dipolar character of the CoNP tip, where the size modulation of the Au and Au@CoNP tips is analogous to control of comonomer reactivity ratios in classical copolymerization processes.
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Au@CoNP colloidal monomers that spontaneously polymerize to form dipolar colloidal polymers (Figure 1a). A major motivation in the field of colloidal polymers has been the application of classical polymer science concepts to these systems, such as, the control of macromolecular degree of polymerization, architecture, and composition. To this end, Kumacheva et al. reported on the preparation of Au nanorods (AuNR) with lateral hydrophilic CTAB ligands and hydrophobic polystyrene functionalized termini which, upon exposure to THF:water solutions, were observed to form linear polymers via tip-to-tip interactions.12 Furthermore, the degree of polymerization was shown to be tunable as a function of reaction time.3 Kotov et al. demonstrated the ability to control colloidal copolymer compositions by the formation of alternating copolymers of differently sized AuNPs achieved via polymerase chain reactions (PCR) from the surface of the AuNPs.13 Additionally, segmented copolymers have been recently reported by Chen et al. via the pH-dependent copolymerization of AuNPs functionalized with poly(styrene-
he use of well-defined inorganic nanoparticles as monomeric units to form one-dimensional (1-D) assemblies, termed “colloidal polymers”, has recently gained attention as a route to prepare nanocomposites that possess intriguing optical, catalytic, and electrochemical properties.1−4 Current efforts in this field have focused on the synthesis of new colloidal monomers that possess strong associative and directional noncovalent interactions, which promote selective 1-D polymerizations as opposed to poorly defined network aggregation.5−11 To this end, recent advances in the total synthesis of complex inorganic colloids, coupled with selective methods to promote “colloidal polymerization”, have enabled a wide variety of colloidal polymers to be prepared. These polymers are formed by harnessing noncovalent secondary interactions designed into the monomers, such as nanoparticle (NP) surface associations, electrostatic forces, or dipolar associations. The focus of our work and this report has been the preparation of colloidal monomers incorporating ferromagnetic metallic cobalt nanoparticles (CoNPs) which spontaneously form 1-D colloidal polymers via dipole interactions between CoNP units. Furthermore, we have also shown that noble metal colloidal NPs, such as gold (AuNPs),6,10 can serve as seeds to promote selective deposition and overcoating of a ferromagnetic CoNP shell to form dipolar © XXXX American Chemical Society
Received: July 4, 2016 Accepted: July 21, 2016
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DOI: 10.1021/acsmacrolett.6b00511 ACS Macro Lett. 2016, 5, 950−954
Letter
ACS Macro Letters
need to develop methods to enable copolymerization of colloidal monomers with disparate NP side chains as well as the ability to controllably tune copolymer composition. Herein, we report on the controllable synthesis of colloidal statistical and segmented copolymers incorporating CdSe@ CdS NRs, CdSe@CdS TPs, and bare AuNP units. We demonstrate for the first time that the controllable photodeposition of a single AuNP tip onto both CdSe@Cd NRs and TPs enables deposition of a single dipolar CoNP shell which spontaneously assembles into colloidal random copolymers. Furthermore, we show for the first time that the composition of these co- or terpolymers can be directly controlled by the size of AuNP tips (and subsequently the CoNP shell) on the NRs and TPs to ultimately prepare either random or block-like colloidal copolymers. The ability to control the copolymerization kinetics of these dipolar colloidal monomers by AuNP tip size has previously been unexplored and is analogous to the concept of comonomer reactivity ratios as observed in classical polymer chemistry.19,20 An attractive feature of this system is the ability to directly image copolymer composition using TEM. The general strategy for this work initially required the selective deposition of a single Au@CoNP tip onto CdSe@CdS NRs and TPs. As alluded to previously, in this seven-step colloidal total synthesis, the key step is the photodeposition of a single AuNP tip onto either CdSe@CdS NRs or TPs. These AuNP tips then promote the deposition of a ferromagnetic cobalt shell to form dipolar Au@CoNPs, which act as the polymerizable unit in the colloidal polymerization via N−S dipole interactions (Figure 2). Hence, for the preparation of coand terpolymers, AuNP tipping of CdSe NRs and TPs were conducted in separate steps, blended in solution and then subjected to CoNP deposition. In the first step, CdSe@CdS NRs were prepared using the seeded growth approach of Pyun and co-workers, wherein wurtzite CdSe quantum dots (QDs) are initially prepared (D = 3.8 nm) and used to seed the growth of a CdS NR shell around the QDs.16 For this study, NR lengths around 100 nm were targeted to enable facile discernment of composition via TEM in the resulting colloidal copolymers. Subsequently, these NRs were functionalized with a single AuNP tip using the photodeposition method of Banin et al., wherein the NRs are irradiated in the presence of an Au3+ precursor, amine reducing agent, and ammonium bromide surfactant to result in initial nonselective deposition of small AuNPs (
